Session J18: Jets in Heavy Ion Collisions

On Demand

Droplets of quark-gluon plasma (QGP), an exotic state of strongly interacting quantum chromodynamics matter, are routinely produced in heavy nuclei high-energy collisions. Although the experimental signatures marked a paradigm shift away from expectations of a weakly coupled QGP, a challenge remains as to how the locally deconfined state with a lifetime of a few fm can be resolved. A colored particle that decays mostly within the QGP is the top quark. Here we demonstrate, for the first time, that top quark decay products are identified, irrespective of whether interacting with the medium (bottom quarks) or not (leptonically decaying W bosons). Using lead-lead collision data recorded by the CMS detector, we report evidence for top quark pair production. Dilepton final states are selected, and the cross section is measured from a likelihood fit to a multivariate discriminator using lepton kinematic variables. The measurement is additionally performed considering the jets originating from the hadronization of bottom quarks, which improve the sensitivity to the signal process. The measurements, consistent with each other and the expectations from scaled proton-proton data as well as perturbative QCD, constitute the first step towards using the top quark as a novel tool to probe QGP.   

On Demand

Measurements of the jet substructure in Pb+Pb collisions provide information about the mechanism of jet quenching in the hot and dense QCD medium created in these collisions, over a wide range of energy scales. This talk presents the ATLAS measurement of the suppression of yields of large-radius jets and its dependence on the jet substructure, characterized by the presence of sub-jets and their angular correlations. This measurement is performed using the large Pb+Pb data sample at the center-of-mass energy of 5.02 TeV recorded in 2018 and compared to the result from 2017 $pp$ collisions at the same collision energy. This study of the suppression of inclusive yields of large-$R$ jets brings new information about the evolution of the parton shower in the medium and tests the sensitivity of the jet quenching to the color coherence effects.   

On Demand

In relativistic heavy ion physics, jets are a collection of particles that are emitted from hard scattered partons. They are known to interact strongly within quark-gluon plasma (QGP) produced in heavy ion collisions, a phenomenon known as jet quenching. It has been observed at the LHC that jets' energy deposition and radiation patterns change when they interact with QGP compared to their vacuum baseline. Jet shape observables are expected to be sensitive to these modifications within a jet and its lateral energy distribution. In this study, we utilize data collected by the STAR experiment at RHIC to compare the evolution of jet shape observables in $\sqrt{s_{NN}} = 200$ GeV proton+proton and Gold+Gold collisions. In particular, we focus on the three jet shape observables $p_{T}D$, $LeSub$, and $g$. These measurements allow us to have a better understanding of the jet quenching phenomenon in heavy ion collisions at RHIC. This study offers a complementary look at jet quenching effects.   

On Demand

We present state-of-the-art constraints on the properties of the quark-gluon plasma by performing a comprehensive Bayesian model-to-data comparison of heavy-ion measurements. Soft observables from both RHIC and the LHC are combined into a global Bayesian analysis, enabling us to obtain more reliable constraints on the transport coefficients of QCD, in particular for the temperature dependence of shear and bulk viscosity. We study multiple different mappings of the hydrodynamic fields to hadronic momentum distributions (i.e. viscous effects on particlization) and for the first time quantify the resulting theoretical uncertainty on the Bayesian analysis. Uncertainties originating from modeling the late hadronic rescattering stage of the collisions are also investigated by preliminary comparisons of SMASH and UrQMD as afterburners. The newly obtained Bayesian constraints are validated by comparisons with additional measurements, including high-statistics observables that are currently too challenging to include in Bayesian calibrations.   

On Demand

Jets produced in high energy heavy ion collisions are quenched by the production of the quark gluon plasma. Measurements of these jets are influenced by the methods used to suppress and subtract the large, fluctuating background and the assumptions inherent in these methods. We compare measurements of the background by the ALICE collaboration to PYTHIA Angantyr simulations of Pb-Pb collisions and to a data-driven random background generator. Angantyr over-estimates the predictions for the standard deviations of the energy in random cones at an approximate level of 15{\%}, indicating that fluctuations due to mini-jets and resonances are not negligible in Angantyr. We are able to describe the standard deviation of the energy in random cones in the background generator as a convolution of number and momentum fluctuations in agreement with the form predicted in the ALICE paper. The description works well for an azimuthally isotropic background but slightly underestimates the width of the correlations for a flow-modulated background. We derive the expected impact of flow on the background and compare to the data-driven background generator.   

On Demand

In Relativistic Heavy Ion Collisions at RHIC and LHC an I$_{\mathrm{AA}}$ distribution is the measurement in di-hadron correlations of the ratio of the A$+$A to p$+$p associated away particle transverse momentum$_{\mathrm{\thinspace }}$(p$_{\mathrm{Ta}})$ distributions for trigger particles with a given p$_{\mathrm{Tt}}$. In all published measurements in Au$+$Au collisions at RHIC (c.m. energy 200 GeV) [PHENIX, PRL104, 252301 (2010)], [STAR, PLB760, 689 (2016)] and Pb$+$Pb collisions at LHC (c.m. energy 2.76 TeV) [ALICE, PLB763, 238 (2016)], the I$_{\mathrm{AA}}$ distribution decreases in the range 0\textless p$_{\mathrm{Ta}}$\textless 3 GeV/c and then becomes constant out to the measurement limit p$_{\mathrm{Ta}}$\textless 20 GeV/c. One possibility discussed is that jet fragments with p$_{\mathrm{Ta}}$\textgreater 3 GeV/c are sufficiently inside the jet cone that they don't lose energy by the BDMPSZ method [arXiv:hep-ph/0002198v2]